Windows 7 cuts almost a third off the battery life of some netbooks shipping today with Windows XP, several recent reviews and user reports say.

Laptop magazine reported in its blog on Monday that during a recent test, a Toshiba netbook lost 2.5 hours of battery life when running Windows 7 instead of XP, or about 30% (6:53 for Windows 7, versus 9:24 for XP).

Web site Tom's Hardware found last month that an Acer Aspire One netbook running Windows 7's release candidate lasted 2.5 hours less than when it ran Windows XP Service Pack 3 (5:54 versus 8:28, when both were at a low-power idle state).

Complaints have also surfaced on netbook user forums such as eeeuser.com, for Asus Eee users, AspireOneUser.com, for Acer netbook users, and MSIWind.net, for MSI fans.

The complaints follow gripes that Windows 7 hastens the vampire-like battery drain of running Windows on MacBooks, either in virtualization or via Apple's Boot Camp software. Jury's still out

The reviews are not unanimous. In a late July review comparing the Windows 7 RC versus XP on Asus' long-running Eee PC 1005HA netbook, Legit Reviews found XP to have between a 2% and 8% advantage over Windows 7. And Laptop noted that XP only had a 6% advantage over Windows 7 on an MSI U123 netbook.

But the negative reports are numerous enough that they darken Windows 7's image as being a sleeker and more-efficient reboot of Microsoft's long-running operating system, and cast some doubt on its suitability for netbooks, at least today's models.

Long battery life is one of the key selling points of netbooks, due to their high portability.

Many vendors heavily tweak their netbooks to ensure that they can run a full business day on a single charge, or more.

Microsoft previously promised that Windows 7 would improve laptop battery life by about 11% over Vista.

That would be due to better use of the graphics chip during tasks such as DVD playback, and improvements in the kernel so that CPU can more quickly switch to an idle state when not in use, and generally run more efficiently, says Microsoft (see video from Microsoft's WinHEC keynote last November).

A Microsoft spokeswoman declined to comment about the recent reviews and reports, but did point to a white paper, last updated June 23, 2009, describing to driver developers and hardware engineers how to optimize hardware and components for better battery life under Windows 7.

Of course, battery life for Windows Vista was widely perceived to be worse than under XP, due to its bloated codebase, which prevented Vista from running well on netbooks, as well as the poor availability of Vista drivers for many months after its launch. More reading: